The long term goal of this research program is to identify the signals on axons that determine whether they are ensheathed or myelinated by Schwann cells and to elucidate the downstream pathways they activate in Schwann cell that regulate these distinct phenotypes. We have been characterizing the neuronal growth factor, neuregulin-1 (NRG1) in these events. NRG1 has three major isoforms (types I, II, and III),which differ in their amino terminal sequences and modes of signaling. Types I and II are paracrine signals that are shed from the axon surface by metalloproteinase cleavage whereas type III is a juxtacrine signal retained at the membrane. We have recently found that type III NRG1 levels provide the long sought instructive signal that determines the ensheathment fate of axons and does so by activating PI 3-kinase in Schwann cells, a key signaling pathway we have found to be essential for ensheathment and myelination. To elucidate how type NRG1, and PI 3-kinase, drives Schwann cell myelination and to examine its potential role in myelin maintenance, we are characterizing its function in a myelinating coculture system that faithfully replicates key events in the development of myelinating fibers in the PNS in vivo. In particular, we propose to: i) determine whether the type III NRG1 isoform is specific in promoting myelination or whether other NRG1 isoforms can substitute for its activity, whether its ability to trigger myelination requires that it be tethered to the axon, and the significance of its cleavage by metalloproteinases, ii) characterize which of the downstream effectors of PI 3-kinase are critical to its ability to promote Schwann cell ensheathment and myelination, and iii) determine whether NRG1 signals are required, not only during development, but also to maintain the integrity of the axon-myelinating Schwann cell unit in the adult. These studies should provide major insights into how the axon drives formation of the myelin sheath, which is essential for normal function of the nervous system. They are likely to have important implications for an understanding of the neurologic disability and pathogenesis of peripheral neuropathies, and of other disorders of myelinated fibers, and may lead to the development of new strategies to promote repair and remyelination of such disorders.